This invention relates generally to internal combustion engine fuel systems, and more particularly to the use of fuel injectors to advantageously control combustion chamber pressure through the use of multiple pilot injections.
It is common practice, in diesel internal combustion engines, to divide the fuel injected into the combustion chamber into two injections within the same injection cycle. The reasons for using two injections include to reduce ignition delay, to reduce the generation of nitrogen oxides, and lower combustion noise. It is commonly known that the two injections achieve a reduction of the combustion chamber pressure or cylinder pressure gradient and consequently a slower buildup of pressure in the combustion chamber.
The first or initial injection is typically known as a pilot injection, while the subsequent injection is commonly called a main injection. This two injection approach or process is known, by those of skill in the art, by various names including Split Shot injection, pilot injection, single pilot injection, pilot-main injection, and multiple injection. Additionally, a similar process known as rate shaping involves providing a small fuel quantity during the initial portion of the main injection.
In the pilot injection method, a pilot injection first delivers a small quantity of fuel into the combustion chamber during a predetermined period typically before Top Dead Center (TDC). An ensuing stage delivers the main or primary injection into the combustion chamber. The pilot injection is typically initiated about 25xc2x0 before TDC, while the main injection is typically delivered at about TDC.
However, even the current pilot-main injection approach still results in noticeable combustion noise due to high heat release rates and a high cylinder pressure increase rate. This is especially the case during engine idle conditions. In attempts to reduce combustion noise, some have varied the amount of fuel in the pilot injection and/or have tried to vary the timing of the injection point of the pilot injection. However, even with these methods, combustion noise levels remain high. There is thus a need for a fuel injection method and apparatus that will noticeably reduce combustion noise in a diesel engine at various engine conditions and load levels, including engine idling conditions.
The present invention substantially reduces diesel combustion noise through the introduction of a pre-pilot fuel injection into the combustion chamber at a predetermined time, during a single compression stroke in an engine cycle, i.e., during a single injection cycle, prior to the point where a critical mass would be formed in the cylinder if the injection were made and prior to the pilot and main injection shots. Critical mass is the state in the combustion process wherein all the conditions, including localized air/fuel mixture, temperature and pressure, necessary to support explosive combustion or uncontrolled heat release exist in the combustion chamber. The pre-pilot fuel injections are accomplished through execution of a pre-pilot injection algorithm by an electronic control module which will manipulate existing injectors and injector controls to deliver a pre-pilot fuel injection prior to the pilot injection at a predetermined optimum time before top dead center (TDC).
The injected pre-pilot fuel vaporizes in the combustion chamber and mixes with the air to form a substantially homogenous air/fuel mixture in the combustion chamber prior to the time that the critical mass may be formed. Preferably, at about the point where the cylinder conditions including the pre-pilot shot would reach a critical mass, the pilot fuel may be injected. This pilot shot has the effect of quenching the pre-pilot mixture in the cylinder and altering the conditions in the cylinder so that a critical mass no longer exists for an additional period. The result is that the fuel combusts or burns in a non-explosive and controlled manner such that the heat release rate of the pre-pilot fuel will begin to increase the cylinder pressure in a continuous and smooth manner. The subsequent pilot/main injection will then result in a smooth pressure rise in the combustion chamber due to the presence of increased pressure resulting from the controlled heat release of the pre-pilot injection. The combination of the pre-pilot injection in combination with the well known pilot/main injection results in a smoother and more controlled cylinder pressure rise as the piston travels from a compression stroke through a power stroke. The sudden cylinder pressure rise typically observed in a combustion chamber where there is only a pilot and/or main injection, is substantially eliminated leading to reduced combustion noise.
It is an object of the present invention to provide a method and apparatus for operating an internal combustion engine fuel injection system to provide a pre-pilot injection in order to achieve a reduction in the heat release rate in a combustion chamber that results in a slower, smoother and more controlled build up of cylinder pressure thereby a reduction in ambient combustion noise.
It is an object of the present invention to provide a method and apparatus for operating an internal combustion engine fuel injection system to provide the injection of a pre-pilot fuel injection in order to achieve a reduction in the cylinder pressure increase rate or pressure gradient in a combustion chamber that results in a slower, smoother and more controlled build up of pressure in a combustion chamber and thereby a reduction in combustion noise.
A more specific object of the present invention to provide a method and apparatus for operating an internal combustion engine fuel injection system to provide the injection of a pre-pilot fuel injection in combination with a pilot injection and a main injection wherein the pre-pilot injection is injected during a portion of the compression cycle under cylinder conditions permitting homogenous mixing of the pre-pilot shot prior to combustion in order to achieve a reduction in the cylinder pressure increase rate or pressure gradient in a combustion chamber that results in a slower, smoother and more controlled build up of pressure in a combustion chamber and thereby a reduction in combustion noise.
In addition to the reduction in combustion noise, the invention provides the further advantage that peak cylinder pressure is substantially reduced when pre-pilot injection is used. Since the peak cylinder pressure is a major determinant of the structural design of an engine, in existing engines, the power output or power density may be increased without a structural penalty and new engines may be designed with a lighter weight structure for the same power output or power density.